The picture showed Zachary on the family jet, eating an impressive buffet breakfast on the way to Fiji.

Alexa had posted the pic to her Instagram account and included a link to her Twitter account.

It turns out that Alexa was documenting her every move on Twitter, complete with GPS locations from her phone. She even posted the invite to her high school graduation dinner, with date, time and location, reports Ashlee Vance at Bloomberg BusinessWeek.

Today, the Twitter and Instagram accounts are gone.

Dell spends $2.7 million annually on his family's security, according to proxy statements filed with the SEC. His company provides the security detail and Dell reimburses the company for his family's protection.

Dell is no stranger to Twitter. He's got his own Twitter account with 55,000 followers and he uses it. But here's a bit of advice to all parents of teens out there: Take a look at what your kids are doing online.

If a billionaire tech visionary spending nearly $3 million a year on family security doesn't know what his kids are doing online, you could be missing something obvious, too.